This project is designed to define the inter- and intra-molecular organization of the regulatory proteins in the thin filaments of the heart. The protein-protein interactions of cardiac troponin will be examined and the effect of calcium on these associations determined. Studies will be conducted on amino acid sequence of cardiac TN-C to gain information about homology with skeletal TN-C and about the postulated calcium binding sites. A possible polymorphism of heart tropomyosin will be explored using physicochemical and immunochemical methods. The amino acid side chains of troponin will be chemically modified, and the altered groups which change the protein's calcium binding, and biological functionality will be identified. In addition, attempts will be made to crystallize troponin and/or its subunits as a step in eventually determining the protein's three-dimensional structure.